Sheet conveying apparatus and image forming apparatus

ABSTRACT

In case a belt member of an endless intermediate transfer belt causes an improper running such as a skewed or zigzag movement, the belt member contacts belt contact detecting portions on both ends of a belt control roller and causes friction thereon. A difference in the frictional forces on the belt contact detecting portions causes a pivoting of a rotary axis of the belt control roller about a roller support member formed by a supporting shaft and a bearing at the center, thereby correcting the belt member in running operation to an appropriate position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for guiding a belt, andmore particularly a belt guiding apparatus equipped with a controlmechanism for preventing a belt skewing. The present invention alsorelates to a driving apparatus utilizing a belt guiding apparatusequipped with a control mechanism for preventing a belt skewing, aconveying apparatus utilizing such belt guiding apparatus, and an imageforming apparatus equipped with such conveying apparatus.

2. Related Background Art

There is known an image forming apparatus for forming a color image,utilizing a belt drive apparatus for driving an endless flat belt,called an intermediate transfer belt. In such image forming apparatus,toner images are formed on plural photosensitive drums serving as imagebearing members of plural colors, and, before being transferred onto arecording sheet, at first transferred onto the intermediate transferbelt, and then collectively transferred onto such recording sheet.

Also the belt drive apparatus is employed in a sheet conveying apparatusfor conveying a sheet as a recording medium.

FIGS. 12 and 13 illustrate a prior image forming apparatus utilizing abelt drive apparatus as the sheet conveying apparatus. A conveying belt131 is supported by an idler roller 132, a drive roller 134 and a beltadjusting roller 135 and is rotated in a direction indicated by anarrow. The belt adjusting roller 135 serves to provide the conveyingbelt with a predetermined tension, and to correct a skew of the belt.The conveying belt 131 is provided with comb-like electrodes, parallelto a transversal direction of the belt, which is orthogonal to aconveying direction, and is also provided, on the belt surface, with anintermediate resistance layer for generating an attractive force in acontact area with the sheet. Also on both lateral ends of the conveyingbelt 131, electrostatic means 136 and charge eliminating means 137 areprovided and apply a high voltage in contact with the comb-likeelectrodes to generate an electrostatic attractive force. Thus the sheetis fixed by electrostatic attraction to the conveying belt 131 and isadvanced to an appropriate position corresponding to a recording head107 in an image forming part. As a positional displacement of the sheeton the belt results for example in an unevenness in the image, the sheetis conveyed under an electrostatic attraction onto the conveying belt131 for avoiding such positional displacement.

Also for monitoring a skew movement of the conveying belt 131, opticalbelt detection sensors 138 such as photointerruptors are provided onboth lateral ends of the belt, and a control apparatus 156 shown in FIG.13 detects a skew amount (zigzag movement amount) of the belt, based ona detection signal from the belt detection sensors 138. The beltadjusting roller 135 is rotatably supported by a roller bearing 155,which can be displaced by a motor 157. A drive signal for turning on/offthe motor 157 is supplied from the control apparatus 156.

In such control mechanism for preventing belt skew, the belt adjustingroller 135 is pivoted, as shown in FIG. 15, about an end thereof at thefar side by displacing the other end thereof at the near side, wherebythe rotary axis C-C is displaced with respect to the conveyingdirection. When the belt adjusting roller 135 is not pivoted but remainsparallel to the rotary axes of other rollers, the conveying belt 131moves from a point A0 to a point B0 in FIG. 15 and does not generate adisplacement (skew displacement) in the thrust direction of the roller.When the rotary axis C-C of the belt adjusting roller 135 is pivoted,the conveying belt 131 moves from A1 to B1 to generate a skewdisplacement ΔX1, or from A2 to B2 to generate a skew displacement ΔX2,whereby the conveying belt 131 is corrected by a displacement toward thenear side. The belt detection sensor 138 detects such skew displacementand moves the belt adjusting roller 135 by a set displacement amountthrough the control apparatus 156, thereby appropriately correcting therunning position of the conveying belt 131.

However, a high-speed running of the conveying belt 131 increases askewing speed, thus often leading to a breakage or the like in the belt.FIGS. 14A and 14B show a prior structure proposed for overcoming suchdrawback. Detection members 140 are provided independently rotatably onboth ends of the belt adjusting roller 135, and a gear 144 provided onthe rotary axis is integrally provided so as to move in synchronizationwith the detection member 140. A belt adjusting roller 135 is supportedby a supporting bracket 143, which is provided with a rack gear 143engaging with the gear 144 on the rotary axis of the roller. Thesupporting bracket 143 is mounted on a frame across a tension spring 145for applying a tension to the belt, and a constant tension is applied tothe belt 131 by a spring force applied in the X-direction.

Also Japanese Patent Publication No. H6-99055 discloses a belt driveapparatus having a function of correcting a zigzag movement of the belt.

However, such prior belt skew preventing mechanisms are associated witha following drawback that has to be resolved. In case the rotary axisC-C of the belt adjusting roller 135 is angularly displaced as shown inFIGS. 14A and 14B, for executing the skew control of the intermediatetransfer belt 131, the belt adjusting roller 135 is subjected to a forcein the thrust direction, thus inducing an abrasion between the roller135 and the bearing therefor by the frictional force in the thrustdirection.

SUMMARY OF THE INVENTION

In consideration of the foregoing, an object of the present invention isto provide a sheet conveying apparatus and an image forming apparatus ofa high reliability, capable of preventing an endless flat belt supportedby plural conveying rollers, from a skew movement or a zigzag movementin the course of rotation thereof, thereby preventing an abrasion or adamage resulting from mutual friction of the components.

The above-mentioned object can be attained, according to the presentinvention, by a sheet conveying apparatus including an endless belt forconveying a recording medium sheet, a belt control roller constitutingat least one of plural rollers for supporting the belt and serving tocorrect an improper running of the belt cause by skew and zigzagmovements thereof, a roller support member for supporting the beltcontrol roller in an angularly variable manner about a fulcrum at alongitudinal center of the roller, and first and second belt contactdetecting portions provided on both end portions of the belt controlroller so as to come into contact with and to be subjected to a frictionby the belt itself in case of an improper running thereof, wherein thebelt control roller causes an angular change of a rotary axis thereofabout the roller supporting member according to a difference between thefrictional forces of the first and second belt contact detectingportions.

The present invention also provides an image forming apparatus includinga sheet conveying apparatus of the above-described constitution and animage forming part for forming an image on a sheet conveyed from thesheet conveying apparatus.

In the sheet conveying apparatus of the present invention, in case theflat belt causes an improper running such as a skew or zigzag movement,the both lateral ends of the belt itself contact either one, oralternately contact both, of the first and second belt contact detectingportions, provided on both end portions of the belt control roller, andthe belt control roller moves with an angular change in the rotary axisthereof according to the difference of the frictional forces in suchcontacts, thereby correcting the belt, in an improper running, to arunning in a proper position. It is thus rendered possible to preventthe abrasion resulting from the friction between the components such asthe belt itself and the roller bearing.

Also in the image forming apparatus of the present invention,particularly in case the flat belt is utilized as an intermediatetransfer belt, a sheet conveying apparatus having the above-describedfunction allows to suppress a displacement of the intermediate transferbelt in the running thereof, whereby an exact and stable image transferis made possible onto the surface of such belt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an ink jet recording apparatusas an embodiment of an image forming apparatus of the present invention;

FIG. 2 is a perspective view showing a conveying belt provided in asheet conveying apparatus of this embodiment;

FIG. 3 is a cross-sectional view showing a relationship between theconveying belt and an image forming part in this embodiment;

FIGS. 4A and 4B are cross-sectional views of the conveying belt of thisembodiment;

FIGS. 5A, 5B and 5C are respectively an elevation view, a plan view anda lateral view of a belt control roller in this embodiment;

FIG. 6 is a lateral view schematically showing a function of the beltcontrol roller of this embodiment;

FIGS. 7A, 7B and 7C are respectively an elevation view, a plan view anda characteristic chart of the belt control roller of this embodiment;

FIGS. 8A, 8B and 8C are schematic views showing operations of a zigzagmovement control in this embodiment;

FIGS. 9A and 9B are respectively an elevation view and a plan view ofthe belt control roller in this embodiment;

Figs. 10A and 10B are views showing an embodiment with a wrap angle ofabout 90°;

FIG. 11 is a view schematically showing a zigzag movement;

FIG. 12 is a perspective view showing an intermediate transfer belt anda belt control roller in a prior structure;

FIG. 13 is a cross-sectional view showing the relationship between anintermediate transfer belt and an image forming part in a priorstructure;

FIGS. 14A and 14B are respectively an elevation view and a lateral viewshowing another prior belt skew preventing mechanism; and

FIG. 15 is a view schematically showing a zigzag movement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the sheet conveying apparatus and the image formingapparatus of the present invention will be explained by respectivepreferred embodiments, with reference to the accompanying drawings.Embodiments will be given in an ink jet recording apparatus and a sheetconveying apparatus to be equipped therein, as a specific example of theimage forming apparatus capable of evidently reflecting the purport ofthe present invention.

Referring to FIG. 1, an ink jet recording apparatus 1 is provided, in amain body thereof, with a sheet conveying apparatus including a sheetfeed part 2, a conveying belt 3 and a sheet discharge part 4. Arecording head 7 constituting recording means forms a color image on asheet P or a recording medium, conveyed by the conveying belt 3. Thesheet conveying apparatus is illustrated in an example including, inaddition to an automatic sheet feeding device constituted of members anddevices explained above, a manually-fed sheet feeding device.

The sheet feeding part 2 is provided with a pressure plate 21 forsupporting a stack of sheets P in a cassette body 23, and a sheet feedroller, which is rotatably supported and serves to pick up and advancethe sheets P one by one by a frictional force, to the exterior of thecassette. The pressure plate 21 is pivotably supported, at a rear endportion thereof in a sheet advancing direction, in the cassette body 23by a rotary shaft. A pressure spring 24 mounted on the bottom side ofthe pressure plate 21 urges an uppermost sheet P toward the sheet feedroller 22. Also in a part of the pressure plate 21 opposed to the sheetfeed roller 22, there is provided a separation pad of a material of ahigh friction coefficient, such as artificial leather, in order to avoidso-called superposed advancing, in which plural sheets P are advanced ina superposed state.

The cassette body 23 is further provided with a separating finger, whichcovers one of the corners of the sheet P, for enabling separation of asingle sheet P, and a release cam for releasing the contact of thepressure plate 21 and the sheet feed roller 22 (both members beingomitted from the illustration). In a stand-by state, the release campresses the pressure plate 21 down to a predetermined position, wherebythe pressure plate 21 is separated to a position not in contact with thesheet feed roller 22.

A driving power from a drive roller 34 is transmitted, for examplethrough gears, to the sheet feed roller 22 and the release cam. Thus thepressure plate 21, upon being released from the release cam, is liftedto bring the sheet P in contact with the sheet feed roller 22, which isrotated to pick up the uppermost sheet P to initiate the sheet feeding.The sheet P is separated by the separating finger one by one and isadvanced to the conveying belt 3. The sheet feed roller 22 rotates untilthe sheet P is advanced to the conveying belt 3, whereupon the stand-bystate is resumed in which the sheets P are separated from the sheet feedroller 22 and the driving power from the drive roller 34 is turned off.Also a sheet feed roller 90 of a manual-insert sheet feed device feeds asheet P, set on a manual insertion tray 91, to the conveying belt 3 inresponse to a record command signal.

The sheet P, conveyed from the sheet feed part 2, impinges, at a leadingend thereof, on a nip of a pair of registration rollers 44 positioned infront of the conveying belt 3, and is stopped in a state forming a loopof a predetermined amount, thereby being corrected from an eventualskewed position. Then, in response to a print start command signal froma main controller (not shown) in the main body of the apparatus, theregistration rollers 44 start to rotate to advance the sheet P to theconveying belt 3.

A platen 30 supports a belt member 31 of the conveying belt 3 in a flatstate from the inner side, thereby inhibiting a downward displacementthereof. The platen 30 serves to assist that the sheet P on the beltmember 31 is advanced to appropriate positions corresponding torespective color recording heads 7 (Y, M, C, K) of the image recordingpart (image forming part). Also in a position opposed to the idlerroller 32, an attraction roller 33 is provided in contact with the beltmember 31, so as to be rotated by the movement thereof. The attractionroller 33 is pressed to the belt member 31 by a spring (not shown),thereby guiding the sheet P to the recording head. Also at an upstreamside of the conveying belt 3, there are provided an upper guide 27 and alower guide 28 for guiding the sheet P to the registration rollers 44.At a downstream side of the idler roller 32, in the conveying directionof the recording sheet, a recording head 7 is provided as recordingmeans for recording an image according to image information.

The sheet P advanced by the registration rollers 44 is conveyed by theidler roller 32 and the attraction roller 33, and the leading end ofthus conveyed sheet P is detected by a PR sensor lever (not shown)thereby determining a recording position on the sheet P.

The recording head 7 is constituted of an ink jet recording head of linetype, in which plural nozzles are arrayed along a direction orthogonalto the conveying direction of the sheet P. In a sequential order fromthe upstream side in the conveying direction of the sheet P, recordingheads of respective colors 7K (black), 7C (cyan), 7M (magenta) and 7Y(yellow) are arranged with predetermined gaps therebetween. Theserecording heads 7K, 7C, 7M and 7Y are supported by mounting on a headholder 7A. These recording heads execute a heating of ink to induce afilm boiling therein, thereby discharging the ink from the nozzles ofthe recording heads utilizing a pressure change caused by a growth or acontraction of a bubble generated by such film boiling, and thus formingan image on the sheet P. The recording head 7 is made adjustable as to adistance (gap) between the nozzle face and the sheet P in the course ofa recording operation. In a non-operating state, the recording head 7 islifted and a cap 8 slides to cover the nozzle face of the recording head7, thereby preventing the ink from solidification.

The sheet discharge part 4 is constituted of a sheet discharge roller 41and a spur 42. The sheet P after image recording by the recording head 7is pinched and conveyed by the sheet discharge roller 41 and the spur42, thus being discharged to a sheet discharge tray 43. The sheetdischarge roller 41 is driven by a driving power transmitted from thedrive roller 34 through transmission means. The spur 42, in order to runon the printed surface after recording, has a small contact area withthe sheet P, thereby not smearing or perturbing the image recorded onthe sheet P even in contact with the printed surface after recording.

As shown in FIGS. 2 and 3, the conveying belt 3 has an endless beltmember 31 for conveying the sheet P thereon in an attracted state. Thebelt member 31 is supported by plural rollers including an idler roller32, a drive roller 34 and a belt control roller 35, and is rotated by adriving power from the drive roller 34. The idler roller 32 and thedrive roller 34 are rotatably supported on a frame 39 of the apparatus.

The belt control roller 35 is a principal member of a belt runningadjusting mechanism featuring the present invention, and exerts afunction as a tension roller for providing the belt member 31 with anappropriate tension, and a function of correcting an improper running inthe belt member 31, such as a skew movement or a zigzag movement. Thebelt control roller 35 is rotatably supported, at both ends thereof, bya control roller frame 71 which is provided in the frame 39 of theapparatus so as to be capable of a rocking motion.

Now referring to FIGS. 5A to 5C, first and second belt contact detectingportions 70 a, 70 b of a cylindrical shape are provided, unrotatably, onboth ends of the control roller frame 71. The belt control roller 35 isrotatably supported, across such belt contact detecting portions 70 a,70 b, by the control roller frame 71. The belt control roller 35 isrotated by the rotation of the belt member 31. The belt contactdetecting portions 70 a, 70 b have a cylindrical shape of a curvaturesame as that of the roller, in order to generate a frictional force uponcontacting the belt member 31, and, in case the belt member 31 causes askew movement or a zigzag movement, it comes into contact with either ofthe belt contact detecting portions 70 a and 70 b. A surface area, inthe belt contact detecting portions 70 a, 70 b, capable of contactingthe belt member 31, has a frictional coefficient sufficient forgenerating a power for displacing the belt control roller 35 for a skewcontrol of the belt member 31 to be explained later. In the presentembodiment, the belt control roller 35 is formed by abrasion-resistantrubber, having a frictional coefficient k of 0.2-0.5 to the belt member31.

Also as shown in FIG. 5C, end shafts 35 a of the belt control roller 35,protruding outwards from the belt contact detecting portions 70 a, 70 bare rotatably supported by bearings provided in movable pieces 35 b. Themovable piece 35B is supported, slidably in a direction B, by a splitguide portion provided at each end of the control roller frame 71.Springs 72 urges the movable pieces 35B in a direction B, therebyproviding the belt member 31 with a tension of 1.0-15 kgf (9.8 N-147 N).In the present embodiment, a stable and highly precise conveyingoperation can be realized by a tension of 4.0 kgf (39.2 N). The controlroller frame 71 is provided with a supporting shaft 73, at a center of aroller width, in the thrust direction of the belt control roller 35. Thesupporting shaft 73 is pivotably supported by a bearing provided in abracket 74, so as to support the belt control roller 35 in an angularlyvariable manner. The bracket 74 is mounted on the frame 39 of theapparatus. The belt control roller 35 is capable of a rocking motion indirections X1, X2 shown in FIG. 6, about the supporting shaft 73(displacing fulcrum). The first and second contract detection portions70 a, 70 b are provided at positions of a same distance from the centerof the supporting shaft 73.

Also compression springs 75 are provided as urging means between bothends of the control roller frame 71 and the frame 39, thus urging thebelt control roller 35 toward a stationary position where the rotaryaxis thereof becomes parallel to the rotary axes of the drive roller 34and the idler roller 32. The compression springs 75 serve to regulate,in a zigzag movement control to be explained later, a magnitude of thefrictional forces generated by the belt contact detecting portions 70 a,70 b for displacing the belt control roller 35. However, the compressionsprings 75 are not essential in the zigzag movement control.

The drawings illustrate an example in which the belt member 31 is woundby a wrap angle of about 180° on the belt control roller. However, suchwrap angle is not restrictive, and a same control theory is applicablealso in case of a wrap angle of about 90° as shown in FIGS. 10A and 10B.However, adjustments on the frictional coefficient and the like arenecessary, because the generated frictional force becomes different.

In case of a belt wrap angle of 180°, as shown in FIGS. 7A-7C and 8A-8C,a tension t of 4 kgf (39.2 N) is given by the tension springs 72 to thebelt member 31 of a width for example of about 350 mm. A drag N, actingon a surface in contact with the belt control roller 35, is constant inany position because of the cylindrical shape thereof, thus providing arelation N=t. When the belt member 31 is driven in such state, the beltcontrol roller 35 rotates by being driven by the belt member 31. Thebelt contact detecting portions 70 a, 70 b, having an external shapesame as that of the belt control roller 35 but provided unrotatably,generates a frictional force upon contact with the moving belt member31. Such frictional force is represented by f1=μN1, wherein p is afriction coefficient between the rear surface of the belt and thecontact surfaces of the belt contact detecting portions 70 a, 70 b, andis generated in a tangential advancing direction in the contact point.The belt contact detecting portions 70 a, 70 b, being mounted on bothends of the control roller frame 71 pivotable about the supporting shaft73, are movable in a rotating plane perpendicular to the supportingshaft 73. A force F, for displacing the belt control roller 35, can beobtained by integrating a component, along such rotating plane, of thefrictional force generated in the contact point of the belt contactdetecting portions, over a contact area.

By employing a ball bearing in the bearing 74 for the supporting shaft73, the control roller frame 71, which supports the belt member 31 of awidth of 350 mm under a tension of 4 kgf (39.2 N), can be controlled bya force F of 10 kgf (98 N) which is a component in the rotating plane ofthe frictional force generated in the belt contact detecting portions.As the moving force F is variable depending on the tension, theresistance in the bearing, the belt width and so on, it is important tooptimize the frictional forces in the belt contact detecting portions70. In consideration of the durability, the friction in the belt contactdetecting portions 70 is preferably made smaller, so that the force Frequired for the displacement should be made as small as possible.

When the belt member 31 has an effective width L equal to or smallerthan a length Lμ of the belt control roller 35, and when a zigzagmovement is generated as shown in FIG. 7A or 9A, the belt contactdetecting portions 70 a, 70 b on both ends of the belt control roller 35do not contact the belt member 31 and do not generate a frictionalforce. On the other hand, in case of a zigzag movement, when the beltmember 31 is skewed to the left as shown in FIG. 9B, it comes intocontact with the belt contact detecting portion 70 a, which thusreceives, from the belt member 31, a frictional force Fa in theadvancing direction thereof. The frictional force Fa generates a momentof clockwise rotating the belt control roller 35 about the centercorresponding to the supporting shaft 73. When the belt control roller35 is pivoted clockwise by an angle corresponding to the frictionalforce Fa, the belt member 31 guided by the belt control roller 35 movesto the right whereby the skew is corrected. When the belt member 31 isskewed to the right, the belt contact detecting portion 70 b at theright side receives, from the belt member 31, a frictional force Fb inthe advancing direction thereof. The frictional force Fb causes acounterclockwise pivoting of the belt control roller 35, whereby thebelt member 31 is displaced to the left to correct the skew.

In an embodiment shown in FIG. 7B, the effective width L of the beltmember 31 and the distance Lμ between the belt contact detectingportions 70 a, 70 b on both ends of the roller have a relationship L≧Lμ.When the belt member 31 is in an appropriate position relative to therollers, the frictional forces received by the belt contact detectingportions 70 a, 70 b from the belt member 31 are mutually equal. However,when the belt member 31 is skewed to either side, the belt contactdetecting portion 70 in such skewed side shows an increased contact areawith the belt member 31, whereby the frictional force received from thebelt member increases in proportion to the distance of displacement. Onthe other hand, the belt contact detecting portion 70 on the other sideshows a decreased contact area with the belt member 31, whereby thereceived frictional force decreases. As a result, the frictional forcesacting on the left and right belt contact detecting portions 70 a, 70 bbecome mutually different, thus generating a moment for pivoting thecontrol roller frame 71 in such a direction as to correct the skew ofthe belt member 31. FIG. 7C is a chart showing a relationship between adisplacement amount of the belt member 31 and a force F for pivoting thecontrol roller frame 71. In FIG. 7C, the slope of the line is determinedby the frictional force generated between the rear surface of the beltmember 31 and the belt contact detecting portions 70 a, 70 b.

As explained in the foregoing, the zigzag movement control of thepresent embodiment can correct the displacement of the belt member 31 bya force, of which magnitude always corresponds to the positionaldisplacement of the belt member 31. Also in case the frictionaldurability is lowered between the belt member 31 and the belt contactdetecting portions 70 a, 70 b, the slope of the line becomes smaller,thereby expanding a control impossible range, since the control is notpossible unless a force exceeding a pivoting resistance of thesupporting shaft 73 is obtained. Such situation can however be avoidedby selecting the distance Lμ between the belt contact detecting portions70 a, 70 b in anticipation of such situation.

Now reference is made to FIG. 11 for reconfirming the behaviors in thezigzag movement control. In FIG. 11, 35 indicates the belt controlroller, and it is assumed that the belt member runs the front side ofthe belt control roller, with respect to the plane of the drawing.

In the belt control roller 35 in a state without a zigzag movement, thebelt member comes into contact with the belt control roller 35 at apoint A0, and remains in contact to a point B0 wherein the belt memberleaves the roller. In such case, there is not generated a force fordisplacing the belt member in a direction orthogonal to the conveyingdirection.

When the belt member is skewed to the left side, the belt member runs,as shown in FIG. 9B, slipping on the surface of the belt contactdetecting portion 70 a. In response, a frictional force Fa acts on thebelt contact detecting portion 70 a, thereby shifting the belt controlroller 35 to a position 35A shown in FIG. 11. As a result, a point onthe belt member 31, coming into contact with the belt control roller 35at a point A1 in FIG. 11 leaves the belt control roller at a point B1.Therefore, when there is no slippage in the thrust direction, the beltmember 31 is rightward displaced by a distance L1, perpendicularly tothe conveying direction. Thus the skew of the belt member to the leftside can be corrected.

On the other hand, when the belt member skews to the right, a frictionalforce Fb acts from the belt member 31 to the belt contact detectingportion 70 b, thereby shifting the belt control roller 35 to a position35B shown in FIG. 11. As a result, a point on the belt member 31, cominginto contact with the belt control roller 35 at a point A2 in FIG. 11leaves the belt control roller at a point B2. Therefore, the belt member31 is leftward displaced by a distance L2, thus correcting the skew ofthe belt member to the right side.

Thus, the belt member 31, in an eventual zigzag movement, can always bereturned to the normal position.

It is thus possible to achieve a control by a small difference in thefriction, utilizing a frictional force generated in either of the beltcontact detecting portions 70 a, 70 b provided on both ends of the beltcontrol roller or utilizing a difference in the frictional forcesgenerated in both detection portions, and to realize a stabilizedconveying operation for the recording sheet P, under a constantmonitoring of the skew in the belt 3 and a stable rotation of the beltmember 31.

FIGS. 4A and 4B are partial cross-sectional views of the belt member 31.The belt member 31, rotated while supporting the sheet P by attraction,is provided with electrodes 60 a, 60 b which are formed by conductivemetals arranged in such a pattern as to generate a strong electrostaticattractive force, and which are protected by a laminate structure of abase layer 62 and a surface layer 61. The base layer 62 is formed by asynthetic polymer resin such as of polyethylene or polycarbonate, whilethe surface layer 61 is formed by a synthetic resin such as afluorinated resin, controllable in resistance for generating an optimumelectrostatic force, and these layers are formed in an endless beltshape. These layers are mutually adjoined for example with an adhesivematerial or by thermal fusion. The electrodes 60 a, 60 b are formed inmutually opposed comb-like patterns with plural teeth arranged in adirection perpendicular to the conveying direction of the belt. On bothlateral end portions of the belt member 31, the surface layer 61 iseliminated to expose the electrodes 60 a, 60 b for enabling powerfeeding brushes 36 to apply a high voltage.

The base layer 62 constituting the belt member 31 is formed by winding asheet (20 μm) constituted of a thermal plastic resin and a thermosettingresin by plural turns (5 turns), followed by thermal fusing, therebyobtaining a sheet of a thickness of 100 μm. Then a thermosetting surfacematerial is wound also in plural turns and thermally fused to completethe belt. During such process, electrodes 60 are formed, in the state ofthe base layer sheet, in predetermined positions thereon, whereby suchelectrodes are provided in a position predetermined within the thicknessof the belt. The electrodes may be formed by printing a conductivepaint, or by depositing a conductive resin and executing thermal fusioncollectively. Such laminate structure improves uniformity of the beltafter formation and secures a stable precision.

A voltage of about 0.5-10 kV is applied to the power supply brushes 36which is in contact with the belt member 31, thereby generating anattractive force in the belt member 31 in the recording positions belowthe recording heads 7. The power supply brushes 36 are connected to ahigh-voltage source (not shown) for generating a predetermined highvoltage. At the sheet discharge, the charge of the electrodes 60 iseliminated by the charge eliminating brush 37 whereby the attractiveforce is lost and the sheet is separated and advanced to the sheetdischarge part 4.

The present invention is not limited to the embodiments described above,but may also be realized in other embodiments or in modifications orvariations of such embodiments within an extent not departing from thescope of the invention.

This application claims priority from Japanese Patent Application No.2005-167502 filed on Jun. 7, 2005, which is hereby incorporated byreference herein.

1. A sheet conveying apparatus comprising: an endless belt for conveyinga recording medium sheet; a belt control roller which constitutes atleast one of plural rollers supporting the belt and serves to correct animproper running of the belt caused by a skew or zigzag movementthereof; a roller support member for supporting the belt control rollerin an angularly variable manner about a fulcrum at a longitudinal centerof the roller; and first and second belt contact detecting portionsprovided on both end portions of the belt control roller so as to comeinto contact with and to be subjected to a friction by the belt itselfin case of an improper running thereof; wherein the belt control rollercauses an angular change of a rotary axis thereof about the rollersupporting member as a fulcrum according to a difference between thefrictional forces of the first and second belt contact detectingportions.
 2. A sheet conveying apparatus according to claim 1, whereinthe first and second belt contact detecting portions are provided at asame distance with respect to the roller support member.
 3. A sheetconveying apparatus according to claim 1, further comprising urgingmeans which urges a rotary axis of the belt control roller toward a baseposition where the rotary axis becomes parallel to those of the pluralrollers.
 4. An image forming apparatus comprising a sheet conveyingapparatus according to claim 1, and an image forming part for recordingan image on a sheet conveyed by the sheet conveying apparatus.
 5. A beltguiding apparatus comprising: a roller maintained in contact with anddriven by a moving belt; a support member for rotatably supporting theroller; support means which pivotably supports the support member insuch a manner that the roller can change a direction of a rotary axisthereof; a first friction member so provided as to pivot integrally withthe support member and receiving, from the belt, a frictional forcewhich becomes larger as the belt is displaced from a predeterminedposition to a first lateral direction; and a second friction member soprovided as to pivot integrally with the support member and receiving,from the belt, a frictional force which becomes larger as the belt isdisplaced from the predetermined position to a second lateral directionopposite to the first lateral direction; wherein the support memberpivots together with the first and second friction members, by thefrictional force which the first or second friction member receives fromthe belt, to change the direction of the rotary axis of the rollerthereby correcting the displacement of the belt.
 6. A belt guidingapparatus according to claim 5, wherein the first and second frictionmembers are unrotatably fixed to the support member.
 7. A belt guidingapparatus according to claim 5, wherein the first friction member has alarger contact area with the belt when the belt is displaced from thepredetermined position to the first lateral direction.
 8. A belt guidingapparatus according to claim 5, wherein the second friction member has alarger contact area with the belt when the belt is displaced from thepredetermined position to the second lateral direction.
 9. A beltguiding apparatus according to claim 5, wherein the first and secondfriction members are positioned at lateral sides of the roller.
 10. Abelt guiding apparatus according to claim 5, wherein a rotation centerof the support member is positioned at a longitudinal center of theroller.
 11. A belt guiding apparatus according to claim 5, wherein adistance from the rotary center of the support member to the firstfriction member is equal to a distance from the rotary center of thesupport member to the second friction member.
 12. A belt guidingapparatus according to claim 5, wherein a distance from the firstfriction member to the second friction member is equal to or larger thana width of the belt.
 13. A belt guiding apparatus according to claim 5,wherein a distance from the first friction member to the second frictionmember is equal to or smaller than a width of the belt.
 14. A recordingmedium conveying apparatus comprising: a belt for conveying a recordingmedium; plural rollers for supporting the belt; a control rollermaintained in contact with and driven by the moving belt; a supportmember for rotatably supporting the control roller; support means whichpivotably supports the support member in such a manner that the controlroller can change a direction of a rotary axis thereof; a first frictionmember so provided as to pivot integrally with the support member andreceiving, from the belt, a frictional force which becomes larger as thebelt is displaced from a predetermined position to a first lateraldirection; and a second friction member so provided as to pivotintegrally with the support member and receiving, from the belt, africtional force which becomes larger as the belt is displaced from thepredetermined position to a second lateral direction opposite to thefirst lateral direction; wherein the support member pivots together withthe first and second friction members, by the frictional force which thefirst or second friction member receives from the belt, to change thedirection of the rotary axis of the roller thereby correcting thedisplacement of the belt.
 15. An image forming apparatus comprising: abelt for conveying a recording medium; plural rollers for supporting thebelt; recording means which records an image on the recording mediumconveyed by the belt; a control roller maintained in contact with anddriven by the moving belt; a support member for rotatably supporting thecontrol roller; support means which pivotably supports the supportmember in such a manner that the control roller can change a directionof a rotary axis thereof; a first friction member so provided as topivot integrally with the support member and receiving, from the belt, africtional force which becomes larger as the belt is displaced from apredetermined position to a first lateral direction; and a secondfriction member so provided as to pivot integrally with the supportmember and receiving, from the belt, a frictional force which becomeslarger as the belt is displaced from the predetermined position to asecond lateral direction opposite to the first lateral direction;wherein the support member pivots together with the first and secondfriction members, by the frictional force which the first or secondfriction member receives from the belt, to change the direction of therotary axis of the roller thereby correcting the displacement of thebelt.